1. Field of the Invention
The invention relates to a semiconductor structure containing an interface between a region made of a predetermined .alpha.-silicon carbide polytype and an electrically insulating region, wherein the electrical conductivity in the silicon carbide region at the interface can be altered by means of induced charges. Semiconductor structures of this type are disclosed e.g. in "IEEE Electron Device Letters", Vol. 18, No. Mar. 3, 1997, pages 93 to 95. The invention also relates to the use of the aforementioned semiconductor structure.
Silicon carbide in monocrystalline form is a semiconductor material with outstanding physical properties that make this semiconductor material of interest particularly for power electronics, even for applications in the kV range, inter alia on account of its high breakdown field strength and its good thermal conductivity. Since the commercial availability of monocrystalline substrate wafers, especially wafers made of 6H and 4H silicon carbide polytypes, has grown, silicon carbide-based power semiconductor components, such as silicon carbide Schottky diodes, are now also receiving more and more attention. However, the unipolar silicon carbide power MOSFETS known to date still pose problems concerning important properties, such as forward resistance (also known as on-resistance).
Energy gaps of silicon carbide polytypes will be discussed quite frequently below. It is understood here that the energy gap values that are to be used for the comparison have to be determined under the premise of identical preparation, measurement and ambient conditions. The values cited below each apply to room temperature. "Properties of Silicon Carbide", edited by G. L. Harris, INSPEC publishers, London, GB, 1995, pages 74-80, documents the energy gaps at a temperature of 4 K for various silicon carbide polytypes.
IEEE Electron Device Letters, Vol. 18, No. Mar. 3, 1997, pages 93 to 95, discloses a unipolar MOSFET fabricated on the basis of a 6H silicon carbide wafer by double ion implantation (so-called DI-MOSFET). In a DI-MOSFET of this type the current is controlled in a semiconductor region made of silicon carbide with lateral current flow, the so-called channel zone which is situated in the silicon carbide region at an interface between the silicon carbide semiconductor region and an electrically insulating region (e.g. SiO.sub.2) . Afterwards, the current is passed vertically through the component in a second silicon carbide semiconductor region, the so-called drift zone.
The DI-MOSFET disclosed affords an improvement of the forward resistance by comparison with the prior art. Further improvements, especially in the forward resistance, which essentially depends on the conductivity of the channel zone and of the drift zone, are promised in "IEEE Electron Device Letters", Vol. 18, No. Mar. 3, 1997, pages 93 to 95, by a change in material from the 6H silicon carbide polytype to the 4H silicon carbide polytype. This statement is based on considerations which take account only of the (bulk) conductivity in the drift zone. In the case of a vertical current flow in the direction of the crystallographic c-axis, the 4H silicon carbide polytype having a free charge carrier mobility of approximately 800 cm.sup.2 V.sup.-1 s.sup.-1 determining the bulk conductivity clearly has an advantage over 6H silicon carbide, which only has a mobility of approximately 100 cm.sup.2 V.sup.-1 s.sup.-1. However, this does not take account of the influence of polytype selection on the second influencing parameter that is critical for the forward resistance, namely the conductivity in the channel zone, which is determined by geometrical parameters and also essentially by the properties of the boundary layer between the silicon carbide semiconductor region and the electrically insulating region.
Palmour U.S. Pat. No. 5,506,421 discloses a semiconductor structure in the form of a silicon carbide MOSFET with a U- or V-shaped trench. Depending on the configuration of the trench, such a MOSFET is also referred to as a U- or V-MOSFET, respectively. The actual MOS structure is situated within the trench, with an electrically insulating region adjoining the trench wall. Within the silicon carbide region, the electrical conductivity can be altered, and thus a current flow can be controlled, in a channel zone adjoining the lateral trench wall. The preferred silicon carbide polytype for the silicon carbide region of the MOSFET is the 6H polytype. The 3C, 2H, 4H and 15R polytypes are mentioned as alternatives. The trench structure, in particular including the non-planar course of the interface towards the electrically insulating region, adversely affects the behavior of the semiconductor structure described in a particular manner. Thus, high electric field spikes are produced at the corners of the trench, above all in the electrically insulating region. As a result, however, the breakdown behavior in the off-state mode is impaired. In the on-state mode, too, the inclined or perpendicular orientation of the lateral trench walls with respect to the main surface of the silicon carbide region can lead to an unfavorable forward resistance of the channel zone.